improve concurrency in the checkPaths function

[alexandear]

The PR refactors checkPaths implementation.
Previously, we wrote all the packages to the work channel before starting to check them. Now, we check each package as soon as it is added to the work channel.

[kisielk]

Do you have any benchmarks or tests that show this makes an improvement? I'm skeptical there would be any noticable difference especially with a buffered channel.

[alexandear]

Closing as the improvement is not significant.

I ran errcheck on a few large repositories and did not notice any significant improvement in execution time.